Typically, a large number of integrated circuits are formed on a silicon wafer. The silicon wafer is then sliced into individual "semiconductor dies" (also referred to as "semiconductor chips") which are then packaged for use.
A "semiconductor device package" typically includes a semiconductor die having a number of bond pads for the purpose of making electrical connections to the integrated circuitry of the die, and is mounted such that the bond pads are exposed. Bond pads, which provide input/output ("I/O") connections to a die, are typically placed along the edges of the die. Inner ends of conductive lines (also referred to as conductive traces, bond fingers, lead fingers or leads) are disposed around the periphery of the die such that they form an array of connection points surrounding the die. A die is typically mounted or attached in a die-receiving area of the semiconductor device package, that is, the area defined by the inner ends of the conductive lines.
Once the semiconductor die is attached, bond pads of a semiconductor die are connected to the connection points provided by the inner ends of conductive lines in one of several ways. Very thin bond wires, usually formed of aluminum or gold, are often used to connect the connections points on a one-for-one basis with the bond pads on the semiconductor die. Tape-automated and solder bump bonding techniques have been employed to connect to the semiconductor die.
Conductive lines extend outward from the die-receiving area, ultimately ending at the external pins of the semiconductor device package, for interfacing with external circuitry and providing electrical connection therewith. The semiconductor device package may be mounted in a socket on a circuit board, typically with additional components.
In the past, dies have been anchored to die receiving areas in an effort to preclude any movement of the die. This is to prevent the die from moving while forming connections between the die and the connection points of the conductive lines. Various die attach techniques have been used. Typical techniques involve interposing an adhesive substance between a die and the die receiving area. In some instances, other die attach materials have been used which were heated in order to form a bond between the die and the die receiving area. Conventional eutectic die attach techniques deposit a layer of gold on the back of a die and mount the die over a layer of gold-silicon provided in the die receiving area. Such techniques provide a bond formed between the die and the die receiving area when exposed to elevated temperatures above 425.degree. C. Other conventional die attach techniques have employed polymers (such as epoxy materials), glass, silver filled glass, and solders, as die attach materials. Placement of the die attach material and its subsequent heating have been burdensome and laborious tasks. Conventional die attach techniques require a manufacturer to incur the cost of die attach materials, such as gold and silver filled glass, which adds to the manufacturing costs. As a result, conventional die attach techniques have been relatively expensive.
One drawback of conventional die attach techniques has been the requirement of heating a die attach material to cure it, because this step in the process also exposes the semiconductor die to elevated temperatures. Ever decreasing geometries of circuit elements are available on a die. In the past, elevated temperatures have caused those circuit elements to fuse together, rendering the die inoperable. Moreover, the structural integrity of the die itself may be compromised by the thermal stresses applied to the die when the die is exposed to elevated temperatures.
Anchoring a die to a die receiving area poses several problems. Heat is inevitably generated during operation of circuitry on a die. The problem of heat dissipation is especially relevant in semiconductor dies that have a high lead count (e.g., high I/O) or which operate at high speeds, both of which may contribute significantly to the generation of heat by the die. Efforts to deal with heat have included the addition of heat sinks bonded to the die. Semiconductor dies generally expand when heated. The rate of expansion is usually a different rate than the expansion of any of a heatsink or other structure to which the die is bonded. At elevated temperatures (or at temperatures significantly different from the temperature at which the die was attached), such differential rates of expansion can cause mechanical stresses which can crack the die (which is relatively brittle), resulting in complete failure of the die.
Conventional semiconductor die attachment techniques have not been altogether satisfactory, and leave room for significant improvement. A technique for mounting semiconductor dies is needed that overcomes the shortcomings of conventional die attachment techniques.